A dubious poll purporting to show Justin Trudeau trailing in his own Montreal riding has set off alarm bells in an industry already struggling to regain credibility after some spectacular failures to gauge election outcomes.

The poll of voters in Papineau, conducted by CROP and commissioned by the NDP, suggested the Liberal leader was running 11 points behind New Democrat Anne Lagacé Dowson.

It was strategically leaked to some media outlets Thursday a few hours before Trudeau was to face off against NDP Leader Tom Mulcair and Conservative Leader Stephen Harper in a crucial debate on the economy — an event with the potential to determine the outcome of the October 19 federal election.

The immediate objective was clear: to rattle the Liberal leader and put him off his game.

But it also had a longer range goal: to break the three-way logjam in voting intentions by persuading Canadians not to waste their votes on a party whose leader can’t even win his own seat.

The Liberals immediately charged that the poll had over-sampled NDP supporters and under-sampled Liberal supporters. And five reputable pollsters who examined the methodology at the request of The Canadian Press agreed that the survey did indeed seem flawed in a number of ways, skewing the results.

Another survey, conducted by Mainstreet Research for Postmedia and involving a sample more than twice the size of the CROP poll, was released the next day with markedly different results, suggesting Trudeau was running five points ahead of Lagacé Dowson.